High-speed transmission is required in long-distance and middle-distance communication fields, specifically, an FTTH (Fiber to the Home) field and an in-vehicle field, for example. In order to attain the high-speed transmission, an optical fiber cable is used as a transmission medium. Further, high-speed communication is being required also in short-distance communication, e.g., communication within 1 m. In a field of such short-distance communication, performance is also required that is unattainable with the optical fiber cable. Specific examples of the required performance include high density wiring such as a narrow pitch, junction, intersection, and multilayering, surface mounting, integration with an electrical circuit substrate, and bending at a small curvature radius. As one that satisfies these requirements, an optical wiring board is considered to be used that includes an optical waveguide.
Such an optical wiring board is preferred to include, for use of light input and output from the optical waveguide, photoelectric conversion elements such as a light emitting element (e.g., a vertical cavity surface emitting laser (VCSEL)) and a light receiving element (e.g., a photo diode (PD)), and a semiconductor element such as an integrated circuit (IC). In order to drive these elements, it is necessary to provide an electric circuit on, for example, the optical wiring board. Thus, preferred is a photoelectric composite wiring board on which not only an optical waveguide but also an electric circuit is provided.
Such a photoelectric composite wiring board is used not only in place of a printed wiring board (PWB) but also, for example, as a photoelectric composite flexible wiring board that is bendable. Such a photoelectric composite flexible wiring board can be used in place of, for example, a flexible wiring board disposed across a hinge of a compact terminal device, and is drawing attention.
As a material used for forming the optical waveguide, for example, an acrylic resin is known that is widely used for producing an optical fiber. The optical waveguide made of an acrylic resin, however, does not have heat resistance high enough for allowing heat conditions for forming an electric circuit, for example, reflow conditions of lead-free solder at a high temperature. Thus, it is impossible to apply a mounting step including a reflow step, for a case of forming an optical waveguide on a substrate with use of an acrylic resin and further mounting various elements such as a photoelectric conversion element. Therefore, in a case of using such an optical waveguide, it is necessary to dispose the optical waveguide on, for example, a separate substrate on which various elements have been mounted in advance, while accurately positioning a core of the optical waveguide in the order of tens of micrometers. Such a mounting step is so complicated that productivity is limited.
For the reasons described above, a material other than the acrylic resin is required as the material for forming the optical waveguide. Examples of the material other than the acrylic resin include materials described in Patent Literatures 1 and 2.
Patent Literature 1 discloses, as a material used for a core and a clad portion of an optical waveguide element, a polymerizable mixture that includes a mixture containing a reactive oligomer such as an oxetane, and a polymerization initiator capable of initiating polymerization of the mixture with heat or light.
Patent Literature 2 discloses a resin composition for an optical waveguide that contains, as essential components, a cyanate ester prepolymer, a sulfur atom-containing compound, an acrylate monomer, and a photoradical polymerization initiator.
Patent Literature 1 describes that a high-quality optical waveguide element is easily attained. Patent Literature 2 describes that it is possible to provide an optical waveguide that has high heat resistance and a small light transmission loss at a practical level and is thus excellent in physical properties.
The optical waveguide is being required to have higher heat resistance. Specifically, in a case of forming an optical waveguide and then forming an electric circuit, as described above, the optical waveguide is being required to have heat resistance high enough for allowing heat generated when the electric circuit is formed, for example, heat resistance high enough for allowing reflow conditions of lead-free solder at a high temperature.
For the reason described above, a material is required that gives an optical waveguide having higher heat resistance. Specifically, a material is required that gives an optical waveguide higher in heat resistance than optical waveguides formed with use of the materials described in Patent Literatures 1 and 2.